I. Field of the Invention
The current invention relates to wireless communications. More particularly, the present invention relates to the use of beam sweeping techniques to provide greater capacity in a multi-user wireless communication system.
II. Description of the Related Art
A modern day communication system is required to support a variety of applications. One such communication system is a code division multiple access (CDMA) system which conforms to the “TIA/EIA/IS-95 Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System”, hereinafter referred to as IS-95. The CDMA system allows for voice and data communications between users over a terrestrial link. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled “SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS”, and U.S. Pat. No. 5,103,459, entitled “SYSTEM AND METHOD FOR GENERATING WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM”, both assigned to the assignee of the present invention and incorporated by reference herein.
The International Telecommunications Union recently requested the submission of proposed methods for providing high rate data and high-quality speech services over wireless communication channels. A first of these proposals was issued by the Telecommunications Industry Association, entitled “The cdma2000 ITU-R RTT Candidate Submission”, hereafter referred to as cdma2000 and incorporated by reference herein. Methods of transmitting user data (non-voice data) over fundamental and supplemental channels are disclosed in cdma2000.
In a CDMA system, a user communicates with the network through one or more base stations. For example, a user on a subscriber station communicates to a land-based data network by transmitting data on the reverse link to a base station. The base station receives the data and can route the data through a base station controller (BSC) to the land-based data network. The forward link refers to transmission from the base station to a subscriber station and the reverse link refers to transmission from the subscriber station to a base station. In IS-95 systems, the forward link and the reverse link are allocated separate frequencies.
The subscriber station communicates with at least one base station during a communication. CDMA subscriber stations are capable of communicating with multiple base stations simultaneously during soft handoff. Soft handoff is the process of establishing a link with a new base station before breaking the link with the previous base station. Soft handoff minimizes the probability of dropped calls. The method and system for providing a communication with a subscriber station through more than one base station during the soft handoff process are disclosed in U.S. Pat. No. 5,267,261, entitled “MOBILE ASSISTED SOFT HANDOFF IN A CDMA CELLULAR TELEPHONE SYSTEM,” assigned to the assignee of the present invention and incorporated by reference herein. Softer handoff is the process whereby the communication occurs over multiple sectors which are serviced by the same base station. The process of softer handoff is described in detail in U.S. Pat. No. 5,625,876, entitled “METHOD AND APPARATUS FOR PERFORMING HANDOFF BETWEEN SECTORS OF A COMMON BASE STATION”, assigned to the assignee of the present invention and incorporated by reference herein
In existing CDMA systems, soft handoffs are established and torn down based on the strength of base station signals received from subscriber stations. In an IS-95 system, for example, subscriber stations measure pilot strength levels for multiple base stations. When the pilot strength level received at a subscriber station from a base station exceeds a threshold T_ADD, that base station is added to the active set of the subscriber station. The base station is removed from the active set of the subscriber station when the strength of the pilot signal received at the subscriber station from that base station falls below a threshold T_DROP. When strength of the same pilot rises again above threshold T_ADD, the base station is re-added to the active set. Backhaul connections between base stations and their respective base station controllers (BSC's) are typically established and torn down in conjunction with these changes in the active set of each subscriber station. The setting up and tearing down of each such backhaul link requires messaging traffic between base stations and the BSC. It is desirable to minimize the backhaul capacity consumed by this messaging traffic. In IS-95, for example, a pilot is not removed from the active set immediately upon its received signal strength dropping below T_DROP. An additional criteria is applied that the strength of a pilot in the active set must remain below T_DROP for longer than a guard time T_TDROP. Adding this guard time requirement decreases the likelihood that a base station is removed from a subscriber station's active set because of spurious signal level fluctuations.
Given the growing demand for wireless data applications, the need for very efficient wireless data communication systems has become increasingly significant. The IS-95 standard is capable of transmitting traffic data and voice data over the forward and reverse links. A method for transmitting traffic data in code channel frames of fixed size is described in detail in U.S. Pat. No. 5,504,773, entitled “METHOD AND APPARATUS FOR THE FORMATTING OF DATA FOR TRANSMISSION”, assigned to the assignee of the present invention and incorporated by reference herein. In accordance with the IS-95 standard, the traffic data or voice data is partitioned into code channel frames which are 20 msec wide with data rates as high as 14.4 Kbps.
A significant difference between voice services and data services is the fact that the former imposes stringent and fixed delay requirements. Typically, the overall one-way delay of speech frames must be less than 100 msec. In contrast, the data delay can become a variable parameter used to optimize the efficiency of the data communication system. Specifically, more efficient error correcting coding techniques which require significantly larger delays than those that can be tolerated by voice services can be utilized.
The parameters which measure the quality and effectiveness of a data communication system are the transmission delay required to transfer a data packet and the average throughput rate of the system. Transmission delay does not have the same impact in data communication as it does for voice communication, but it is an important metric for measuring the quality of the data communication system. The average throughput rate is a measure of the efficiency of the data transmission capability of the communication system.
In a CDMA communication system, capacity is maximized when the transmission energy of signals is kept to the minimum value that will satisfy reliability performance requirements. The reliability in reception of a signal depends on the carrier-to-interference ratio (C/I) at the receiver. Thus, it is desirable to provide a transmission power control system that maintains a constant C/I at a receiver. Such a system is described in detail in U.S. Pat. No. 5,056,109 (the '109 patent) entitled “Method and Apparatus for Controlling Transmission Power in a CDMA Cellular Telephone System”, assigned to the assignee of the present invention and incorporated by reference herein.
In the '109 patent, a closed loop power control system is described in which the C/I (referred to in the '109 patent as signal to noise ratio) is measured at the receiver and compared to a single threshold value. When the measured C/I exceeds the threshold, a power control command is sent requesting the transmitter to decrease its transmit power. Conversely, when the measured C/I falls lower than the threshold, a power control command is sent requesting the transmitter to increase its transmit power. Because the C/I is not the only factor that determines the reliability of reception of a signal, the '109 patent also describes an outer loop power control system that varies the threshold value in order to satisfy a target reliability.
It is well known that in cellular systems the C/I of any given user is a function of the location of the user within the coverage area. In order to maintain a given level of service, TDMA and FDMA systems resort to frequency reuse techniques, i.e. not all frequency channels and/or time slots are used in each base station. In a CDMA system, the same frequency allocation is reused in every cell of the system, thereby improving the overall efficiency. The C/I that any given user's subscriber station achieves determines the information rate that can be supported for this particular link from the base station to the user's subscriber station. Given the specific modulation and error correction method used for the transmission, which the present invention seeks to optimize for data transmissions, a given level of performance is achieved at a corresponding level of C/I. For idealized cellular system with hexagonal cell layouts and utilizing a common frequency in every cell, the distribution of C/I achieved within the idealized cells can be calculated. An exemplary system for transmitting high rate digital data in a wireless communication system is disclosed in copending U.S. patent application Ser. No. 08/963,386, entitled “METHOD AND APPARATUS FOR HIGHER RATE PACKET DATA TRANSMISSION,” (hereafter the '386 application) assigned to the assignee of the present application and incorporated by reference herein.
It is also well known that much of the signal interference in a loaded CDMA system is caused by transmitters belonging to the same CDMA system. In an effort to increase capacity, cells are often divided into sectors or smaller cells operating at lower power, but such methods are costly and difficult to apply in areas having widely varying signal propagation properties. The data communication system of the present invention provides a way of decreasing the mutual interference between elements in the system without requiring a large number of small cells.